Today's metro networks have evolved from the need to support traditional voice and private line services and, as a result, were optimized to support Time Division Multiplexing (TDM) services. However, the growth of private line services is dominated by access (also called “backhaul”) to packet switches that provide Frame Relay, ATM, IP and Ethernet services. In addition, the dominant link layer used in enterprise networks is Ethernet. Since Ethernet interfaces to network equipment have historically been significantly less expensive than TDM interfaces of similar bandwidth, enterprise customers have an incentive to deploy Ethernet interfaces to their network service provider. TDM interfaces do not lend themselves to efficient transport of bursty packet data, since they have to be provisioned statically to carry the peak traffic and must use the coarse bandwidth granularity offered by the TDM multiplexing hierarchy. Because of the strict partitioning of bandwidth, the current TDM network structure has some obvious limitations:                The metro network has many points where circuits are subject to TDM multiplexing and de-multiplexing. Transport networks have evolved to this structure because the TDM multiplexing hierarchy creates circuit bundles at different rates and it is necessary to demultiplex them to the individual circuits and then to aggregate circuits going to the same destination in order to achieve high link utilization. Three types of Digital Cross-connect Systems (DCSs) have been developed to handle this task, i.e., Narrowband, Wideband, and Broadband DCSs cross connect signals at the DS0, DS1 (or SONET VT-1.5 rate), and DS3 (or SONET STS-1) rate respectively;        There may be many other packet access circuits along the path of a typical circuit, but because of the TDM encapsulation, the network cannot take advantage of statistical multiplexing across these access circuits;        The customer has only a coarse granularity of bandwidths from which to choose, based on the historical TDM multiplexing hierarchy (DS1, DS3, etc.). Thus, customers must purchase a TDM access circuit large enough to accommodate their peak demand, even if this means running it at low utilization;        Packet switches and routers must support channelized TDM interfaces that can demultiplex down to low rates. These functions consume precious space on customer-facing interface cards, which reduces the space for the packet processing and queueing functions that must be performed on these cards. This ultimately increases network cost;        Demand for Ethernet interfaces is growing. To provide Ethernet service in the present network, packets are encapsulated into TDM circuits and homed to a packet or Ethernet switch at the gateway office (a “hub and spoke” architecture). This is an inefficient method to provide Ethernet services.        
Therefore, a need exists for a Packet Aware Transport Network (PAIN) architecture to provide more efficient support for packet services, while continuing to support existing TDM interfaces and service capabilities within a metropolitan area.